WO 96/21568 describes a method for joining a sensor to a glass pane with the aid of a transparent adhesive tape. In this case, a so-called acrylic foam may, for example, be utilized as the join medium. This material rigidly adheres to the glass pane and is very difficult to remove. The mounting of a sensor on a glass pane requires a vacuum and, under certain circumstances, the prior heating of the glass pane in order to obtain a joint without air bubbles. If the sensor emits or receives radiation, it is frequently desired to incorporate an infrared-permeable daylight filter (black appearance). However, a corresponding black coloring of the acrylic foam is not possible. Long-term temperature tests at 100 degrees resulted in yellowing and hardening of the acrylic foam. These aging phenomena have negative effects on the function and the mechanical retention of the sensor. Consequently, the invention is based on a joining medium silicone gel.
The invention aims to disclose a non-hardening, transparent joining medium that can be colored such that it is permeable to infrared radiation and ensures a permanent and superior joining of a sensor to the respective body.
The invention, in principle, consists of utilizing a gel that remains soft. Such a gel is, for example, available on the market under the designation SilGel 612 from Wacker. One preferably utilizes a colorless silicone gel that has a viscosity of approximately 800-1200 mm2/s at 23 degrees C. and a density of approximately 0.96 g/cm3. The index of reaction n025 may have a value of approximately 1.4026 at 25 degrees C. The aforementioned silicone gel is frequently referred to as RTV-2 caoutchouc silicone and is available on the market under the trade name Semicosil 912 from Wacker Chemie GmbH.
The main advantages of this gel can be seen in the fact that it cannot harden, and that it remains a soft, gelatine-like vulcanized material even after a prolonged period of time and after being subjected to heat and insolation for prolonged periods of time. Another important aspect regarding the gel is that it cannot permanently enclose air particles. These air particles are removed from the gel due to their natural movement after a certain period of time. The gel also prevents stress between the housing of the sensor and the glass pane to which the sensor is joined.
One particular advantage of the gel used can be seen in the fact that it can be colored with a dye that is permeable to infrared measuring radiation, with said dye largely acting as a filter for other (light) radiation such that the measuring accuracy of a measuring device (e.g., a rain sensor) that operates with infrared radiation can be increased.
It appears possible to directly join the transmitter and/or receiver (sensor) to the glass pane by means of admixtures to the silicone gel. The present invention also discloses a new method for mounting the sensor on a body, in particular, a windshield, which utilizes the previously described joining medium. In this case, the objective consists of simply pressing the sensor onto the windshield by means of a prestress by utilizing the joining gel. In this respect, it was proposed to elastically join the sensor to the glass pane by means of the gel with the aid of a spring element that is supported on the glass pane, e.g., a spring element that is arranged in the bracket of a rearview mirror which contacts the glass pane. According to the invention, a sensor is particularly suitable for this mounting method. Consequently, the invention, in principle, consists of utilizing a gel of the previously described type as the joining medium for this mounting method, with the transmitter and/or receiver being combined in the form of a transceiver that operates as a rain sensor. The radiation emitted and received by the rain sensor preferably consists of infrared light, with the joining medium being colored with a dye that is permeable to infrared light, in particular, a black dye. Due to this measure, the rain sensor that is usually provided with a dark housing has an entirely black appearance such that the joining medium which is visible through the glass pane cannot be optically differentiated from the rest of the sensor housing.
In order to ensure that the detectors and radiation elements arranged within the sensor and the receiver have exactly the prescribed distance from the glass plane surface, the invention proposes the additional development defined by the projections maintaining an exact distance between the transmitter and/or receiver and the glass pane. In addition, the resulting channels ensure that excess gel is able to escape. Consequently, the gel situated between the sensor surface and the glass pane not only has a predetermined thickness, but the desired density can also be obtained largely independent of the quantity introduced during the mounting process. In this respect, it is possible to apply a slightly larger quantity of the gel than required onto the surface of the sensor which faces the windshield. When the sensor or the sensor housing is subsequently pressed onto the glass pane with its front circumferential edge, the excess gel is able to laterally escape through projections such that the gel is always arranged between the sensor and the glass pane with the same stress and the same density. In order to preserve the density and the stress in the joining medium (gel), the invention proposes the additional development in which as mentioned previously, it was proposed to generate prestress by means of additional springs that are supported on the glass pane. This particular variation can be simplified by utilizing an additional development of the invention which is defined by the combination of characteristics proposes that a holder which acts upon the sensor in a largely rigid fashion is mounted on the glass pane. In this case, the elastic effect is not achieved by means of an additional spring, but rather by the spring effect of the joining medium (gel). Due to this measure, a largely uncomplicated construction for the holder is achieved. In thus case, the wall of the holder is initially connected to the glass pane during the mounting process, whereafter the sensor with its housing and the applied joining medium is inserted into the wall of the holder. Subsequently, the cover is connected to the wall of the holder by exerting a force in the direction of the glass pane. However, other constructions would also be conceivable. For example, the sensor housing can be directly locked in the wall of the holder, or the holder with the inserted sensor (and joining medium) can be mounted (bonded) to the glass pane by exerting a pressing force.
The shape of the wall of the holder is adapted to the outer contour of the sensor. This wall has a particularly simple design if the sensor has an essentially circular shape, i.e., the wall has an annular appearance.
The function of the holder does not necessarily have to consist of providing a support for the sensor relative to the glass pane. The holder may also accommodate electric connections, e.g., in the form of a multiple plug. These electric connections make it possible to connect the sensor to the corresponding electric units during the mounting process. The power supply may also be realized via the electric connections of the holder.
The holder may, in principle, be simply bonded to the glass pane by means of adhesive agents. However, it would also be conceivable to lock the holder on the glass pane by means of locking elements that are cast into the glass pane. A particularly simple arrangement for the holder is proposed in a combination of characteristics in which the holder is shaped out of the glass pane material or cast into the glass pane. For example, a holder that consists of metal or plastic is cast into the glass pane during its manufacture. However, the glass pane material can also be directly shaped into a holder. Accordingly, connecting lines which lead to the holder are not bonded onto the glass pane in this case, but directly cast into the glass pane.
If the quantity of the joining medium situated between the sensor and the glass pane is precisely metered, the sensor can be arranged at a prescribed distance from the glass pane and the joining medium can be provided with the desired density between the sensor and the glass pane, e.g., if the holder and the sensor are subject to strict tolerances. However, in order to allow an additional tolerance, the invention proposes an additional development in which the projections do not necessarily have to contact the glass pane in order to exactly maintain the corresponding distance. It suffices if the projections form channels, through which the excess joining medium can be laterally pressed out due to a predetermined force, i.e., the gel subsequently has a predetermined thickness. The width of the channels consequently influences the stress to which the gel is subjected in the joined state.
The snap-in connection between the holder and the sensor in accordance with the invention can also be utilized for separating this connection if a permissible lateral force is exceeded. In this case, the sensor is freely movable relative to the glass pane such that the risk of injuries due to a projecting sensor is eliminated during an accident